1. Field of the Invention
The present invention relates to electrophotographic image forming apparatuses, such as copiers, printers, facsimile machines, and multifunction products with functions of these machines, and belt devices provided in the image forming apparatuses. The invention particularly relates to belt devices and image forming apparatuses in which a belt member such as an intermediate transfer belt, a transfer conveyor belt, or a photoreceptor belt performs detaching operation.
2. Description of the Related Art
As seen in conventional copiers, printers, and other image forming apparatuses, tandem-type color image forming apparatuses include an intermediate transfer belt (belt device) (for example, see Japanese Patent Application Laid-Open No. 2001-242680).
Specifically, four photosensitive drums (image carrying bodies) are arranged so as to face an intermediate transfer belt, and carry black, yellow, magenta, and cyan toner images respectively formed thereon. The toner images formed on the photosensitive drums are transferred to and superimposed on the intermediate transfer belt. The multi-color toner image carried on the intermediate transfer belt is transferred to a recording medium as a color image.
Such image forming apparatuses employ technologies that, in forming a black image, a photosensitive drum for black is brought into contact with the intermediate transfer belt so as to detach the other photosensitive drums (for yellow, magenta, and cyan) from the intermediate transfer belt (for example, see Japanese Patent Applications Laid-Open No. 2001-242680 and No. 2006-201338). These technologies attempt to make the drums and the belts to have longer life, prevent scattering of toners, and reduce waste of the toners, for example.
Specifically, Japanese Patent Application Laid-Open No. 2001-242680, for example, discloses a technology that, in forming a black image, one of supporting rollers stretching an intermediate transfer belt is moved so as to detach the intermediate transfer belt from the photosensitive drums for the other three colors.
Japanese Patent Application Laid-Open No. 2006-201338, for example, discloses a technology that, in forming a black image, from among four transfer rollers (primary transfer rollers) facing the four photosensitive drums, three of them exclusive of the one for black are moved by an intermediate transfer belt so as to detach the intermediate transfer belt from the photosensitive drums for the other three colors.
Such conventional image forming apparatuses have difficulties in detaching, when forming a black image, the three image carrying bodies other than the one for black with a sufficient distance from the intermediate transfer belt without increasing their apparatus size.
Specifically, in the image forming apparatus as disclosed in Japanese Patent Application Laid-Open No. 2001-242680, the intermediate transfer belt is detached from the photosensitive drums for the three colors by substantially moving one of the supporting rollers stretching the intermediate transfer belt. This arrangement will secure sufficient space between the three photosensitive drums and the intermediate transfer belt. However, the size of the image forming apparatus is increased because sufficient space is necessary for movement of the intermediate transfer belt.
In the image forming apparatus as disclosed in Japanese Patent Application Laid-Open No. 2006-201338, the intermediate transfer belt is detached from the photosensitive drums for the three colors by moving the three transfer rollers other than the one for black. This arrangement will reduce space inside the intermediate transfer device (or image forming apparatus). However, as the intermediate transfer device becomes smaller, it becomes more difficult to secure sufficient space between the intermediate transfer belt and the three photosensitive drums other than the one for black. Specifically, sufficient space cannot be secured between the photosensitive drum closest to the photosensitive drum for black being in contact with the intermediate transfer belt and its adjacent photosensitive drum, with the result that the both members contact each other. This may cause problems in that the photosensitive drums being close to each other and the intermediate transfer belt become worn, and that toner adhered to the adjacent photosensitive drums scatter over the intermediate transfer belt.
These problems are not limited to the belt devices using an intermediate transfer belt as a belt member, and are commonly seen in those using a transfer conveyor belt or photoreceptor belt as a belt member.
Thus, the belt devices that use a transfer conveyor belt facing the four photosensitive drums (image carrying bodies) have difficulties in detaching, when forming a black image, the three photosensitive drums other than the one for black with sufficient space from the transfer conveyor belt without increasing their apparatus size. The belt devices that use a photoreceptor belt facing the four developing rollers (developer carrying bodies) have difficulties in detaching, when forming a black image, the three developing rollers other than the one for black with sufficient space from the photoreceptor belt without increasing their apparatus size.